Carbon monoxide usually is obtained by separation from synthesis gases produced by catalytic conversion or partial oxidation of natural gas, oils or other hydrocarbon feedstock. In addition to carbon monoxide, these gases contain primarily hydrogen and methane but are often contaminated with significant amounts of nitrogen (derived from the feed or added during processing). Conventional cryogenic separation processing leaves nitrogen as an impurity in the carbon monoxide, which, for both environmental and processing reasons, is unacceptable for some uses of carbon monoxide. The problem of nitrogen contamination of carbon monoxide product is becoming an increasing problem with the usage of more marginal feed stock in front end reforming processes. Accordingly, there is a demand for efficient and effective removal of contaminant nitrogen from carbon monoxide-containing feeds.
Prior art processes for the removal of nitrogen from methane-containing synthesis gas usually include the sequential steps of removing hydrogen from the synthesis gas, removing methane from the resultant hydrogen-freed steam, and removing nitrogen from the resultant hydrogen- and methane-freed stream to leave a purified CO product stream. Usually, at least part of the condensation and reboil duty for one or more of those columns is provided by a recycle carbon monoxide heat pump stream
U.S. Pat. No. 4,478,621 discloses such a process for the recovery of carbon monoxide in which synthesis gas feed is partially condensed and the resultant two phase mixture fed to a wash column in which carbon monoxide is scrubbed from the vapor phase by contact with a liquid methane stream to provide CO-loaded methane containing some, typically 3-4%, hydrogen. A CO recycle heat pump stream provides intermediate indirect cooling to the wash column to remove the heat of solution of carbon monoxide in methane. Residual hydrogen is removed from the CO-loaded methane in a stripping column to meet the required carbon monoxide product specification. The hydrogen-stripped CO-loaded methane is separated into nitrogen-contaminated carbon monoxide overheads vapor and methane-rich bottoms liquid in a methane-separation fractionation column in which both overheads cooling and bottoms reboil is indirectly provided by the CO recycle heat pump stream. Nitrogen is removed from the carbon monoxide overheads in a nitrogen/CO fractionation column to provide CO product bottoms liquid. Overheads cooling to the nitrogen/CO fractionation column is indirectly provided by expanded CO product bottoms liquid and bottom reboil is directly provided by the CO recycle heat pump stream.
EP-A-0676373 discloses a similar process for the recovery of carbon monoxide but in which hydrogen is separated from synthesis gas feed by partial condensation. The condensate is separated into nitrogen-contaminated carbon monoxide overheads vapor and methane-rich bottoms liquid in a methane-separation fractionation column. Nitrogen is removed from the carbon monoxide overheads in a nitrogen/CO fractionation column to provide CO product bottoms liquid. Partial condensation of overheads from at least one of said fractionation columns and bottoms reboil to the nitrogen/CO fractionation column are provided by a CO recycle heat pump stream. In one embodiment (FIG. 5), CO product bottoms liquid from the nitrogen/CO fractionation column is further distilled in an argon/CO fractionation column to provide argon-freed CO overheads vapor and an argon-enriched bottoms liquid. Bottoms reboil for the argon/CO fractionation column also is provided by the CO recycle heat pump stream.
The stated characterising feature of the process of EP-A-0676373 is reduction of energy consumption and plant capital cost by providing overheads condensation for only one of said separation columns and refluxing the other of said columns with liquid extracted at an intermediate location of the said column having overheads condensation. However, it does describe a process (FIG. 2) which does not have said reflux feature but partially condenses overheads of both the methane- and nitrogen-separation columns.
DE-A-19541339 discloses a process for removing nitrogen from synthesis gas in which the synthesis gas feed is partially condensed and hydrogen is removed from the condensed fraction in a stripping column to provide a hydrogen-freed CO-rich liquid. Nitrogen is separated from said CO-rich liquid in a nitrogen-separation fractionation column to provide a nitrogen-freed CO-rich bottoms liquid. Part of said nitrogen-freed CO-rich bottoms liquid is vaporized and both the vaporized and remaining (liquid) portions are fed to a methane-separation fractionation column to provide CO product overheads vapor and methane bottoms liquid. Optionally, additional CO is recovered from the hydrogen-rich vapor portion of said partial condensation of the synthesis gas feed by, for example, pressure swing adsorption or membrane separation and processing of the flush gas or membrane retentate.
Reboil to all three columns of DE-A-19541339 is provided by vaporizing a portion of the respective bottoms liquid and returning the vaporized portion to the relevant column. In one embodiment (FIG. 1), heat duty for the reboil of all three columns and condensation duty for reflux of the nitrogen-separation column is provided by a CO recycle heat pump stream, which also directly provides reflux to the methane-separation column. In remaining embodiments (FIGS. 2 & 3), heat duty for the reboil of all three columns and condensation duty for reflux of both the nitrogen- and methane-separation columns is provided by a (nitrogen) closed circuit heat pump stream.
It is an object of the present invention to provide a more cost effective process for separating carbon monoxide from gaseous mixtures containing carbon monoxide and hydrogen and contaminated with nitrogen, especially those which also contain methane.